1. Field of the Invention
The present invention relates to a cooling arrangement in a gas turbine moving blade in which the blade is cooled by a cooling medium supplied from an interior of a rotor into a cooling medium flow path.
2. Description of the Prior Art
A conventional gas turbine moving blade structure will be described with reference to FIGS. 3 and 4. FIG. 3 shows a gas turbine using air as the cooling medium, and illustrating an example of introducing cooling air into the gas turbine moving blade.
As indicated by arrows, cooling air flows through a cooling air pipe 51, passes through a hole 53 in a rotor disc 52, and then flows into a hollow moving blade 54 so as to cool the blade. Meanwhile, reference numeral 55 denotes a combustor and reference numeral 56 denotes an axial compressor. FIG. 4 shows an example of the moving blade having an internal cooling path.
Cooling air entering from a bottom portion of a blade root 71 flows in the direction indicated by the arrows so as to cool the moving blade. That is, cooling air entering from a leading edge side 72A flows in a cooling air flow path having a plurality of cooling fins 73 forming a turbulence promoter so as to cool the moving blade. Finally, the air flows out from a hole A on a blade top portion having a thinned tip portion 74 so that the air merges with the main gas flow.
On the other hand, in the direction indicated by arrows, cooling air entering from a trailing edge portion 72B flows in a cooling air path having a plurality of cooling fins 73. The air cools a blade trailing edge through pin fins 75. Then, the cooling air flows out from a plurality of holes B provided on the blade trailing end so that the cooling air merges with the main gas flow. Further, reference numeral 76 denotes a blade platform.
As described above, the conventional blade is constructed so that cooling air, transferred from the disc to the blade root, is used for cooling the moving blade and is finally discharged into the main gas flow.
In the aforementioned conventional structure, because the cooling air after cooling the moving blade is discharged into the main gas flow, the cooling air is a negative factor in terms of thermal efficiency of the turbine.
Further, in the air-cooled gas turbine, it is possible to enhance thermal efficiency by intensifying the sealing between the main gas flow and the interior of the rotor so as to block a back-flow of hot gas from a sealing portion and to minimize the amount of cooling air flowing into the main gas flow. Thus, an effective structure is that in which only the necessary amount of cooling air is fed to the moving blade so as to cool the blade while other air is fed to the sealing portion.
In recent years, steam has been more often used as the cooling medium instead of cooling air in order to raise the efficiency of the gas turbine. However, in such steam cooling, because extraction steam from a steam turbine composing a combined cycle, steam from a waste heat boiler or the like is used, and a complete elimination of leakage of cooling steam into the gas turbine is necessary for the reasons of the steam cycle such as supply of demineralized water, prevention of lowering of plant thermal efficiency and the like.
Thus, it is important that the cooling medium path is closed-relative to the outside so that only a supply port and a recovery port are provided thereby facilitating production thereof. Thus, the cooling medium path is generally designed so that the cooling medium is supplied from an axial end on the discharge side of the gas turbine rotor and is recovered at the axial end.
In both air cooling and steam cooling, it is important that the moving blade is adequately cooled without allowing the cooling medium to escape. Then, although it is absolutely necessary that in either air cooling or steam cooling, the cooling medium is adequately recovered, the conventional cooling medium path does not have an appropriate structure for reliably recovering the cooling medium.
Another problem associated with the conventional cooling medium path structure concerns a transfer position of the cooling medium between the disc and the blade root, that is, appropriate sealing performance at this location has not been secured due to the internal pressure of the cooling medium, the difference in thermal expansion between the disc and the blade root, the centrifugal force and the like.